CaO-based CO2 Sorbents Research Articles

Exploring the role of surface and porosity in CO2 capture by CaO-based adsorbents through response surface methodology (RSM) and artificial neural networks (ANN)

In this study, synthetic CaCO3 materials were utilized as precursors for CaO-based CO2 sorbents. The investigation examined how various operating parameters—such as synthesis temperature (ST), stirring rate (SR), and surfactant percentage (SP)—impact the properties of the adsorbents. Samples were firstly characterized by X-ray diffraction and Scanning Electron Microscopy (SEM), which revealed that the prevalence of calcite or aragonite crystal phases in the synthetic CaCO3 precursors can be tuned by adequately choosing the dose of surfactant (Triton-X100®), so that it can be used as a crystal habit and growth modifier. The calcination process applied to the CaCO3 precursors leads to the formation of partially sinterized cubic crystals of CaO, accompanied by minor quantities (< 5 %) of additional compounds like Ca(OH)2 or CaSO4. Specific surface area (SBET) and porosity were determined by measuring the N2 adsorption isotherms. A CaCO3 sample with an unprecedented value of SBET as large as 116 m2/g was prepared operating under optimal conditions. SBET and pore volumes were successfully correlated with the CO2 uptake capacity of the samples. SBET is more influential for experiments carried out under diluted CO2 atmosphere. When pure CO2 is used, the influence of meso- and micropore volumes (Vme and Vmi) is clearly predominant, which suggests that in this latter case diffusion through the porous texture of the samples plays a more remarkable role. A double-way approach through Response Surface Methodology (RSM) and the use of Artificial Neural Networks (ANNs) has been used to analyze the CO2 uptake capacity of the samples. Within the operational interval, excellent results were obtained for pure and diluted CO2 flow, and RSM and ANNs have demonstrated to be a very efficient tool to correlate the behavior of the CaO-based materials as CO2 sorbents with the surface area and pore volumes of the samples. Valuable information on (i) the importance of the different factors under study; (ii) their influence on the surface and porosity of the CaO-derived sorbents; and (iii) the subsequent CO2 capture performance of the sorbents has been obtained. The results suggest that four parameters have a statistically significant influence on CO2 uptake. These parameters are SR, the square of SR, its interaction with SP, and the square of SP. Additionally, the study assessed the stability of the CaO-based sorbents over 11 consecutive calcination-carbonation cycles. By adequately choosing the synthesis strategy and conditions, an almost negligible shrinkage effect can be achieved, resulting in a more sustained uptake capacity throughout the cycles.

Of Glasses and Crystals: Mitigating the Deactivation of CaO-Based CO2 Sorbents through Calcium Aluminosilicates.

CaO-based sorbents are cost-efficient materials for high-temperature CO2 capture, yet they rapidly deactivate over carbonation-regeneration cycles due to sintering, hindering their utilization at the industrial scale. Morphological stabilizers such as Al2O3 or SiO2 (e.g., introduced via impregnation) can improve sintering resistance, but the sorbents still deactivate through the formation of mixed oxide phases and phase segregation, rendering the stabilization inefficient. Here, we introduce a strategy to mitigate these deactivation mechanisms by applying (Al,Si)Ox overcoats via atomic layer deposition onto CaCO3 nanoparticles and benchmark the CO2 uptake of the resulting sorbent after 10 carbonation-regeneration cycles against sorbents with optimized overcoats of only alumina/silica (+25%) and unstabilized CaCO3 nanoparticles (+55%). 27Al and 29Si NMR studies reveal that the improved CO2 uptake and structural stability of sorbents with (Al,Si)Ox overcoats is linked to the formation of glassy calcium aluminosilicate phases (Ca,Al,Si)Ox that prevent sintering and phase segregation, probably due to a slower self-diffusion of cations in the glassy phases, reducing in turn the formation of CO2 capture-inactive Ca-containing mixed oxides. This strategy provides a roadmap for the design of more efficient CaO-based sorbents using glassy stabilizers.

CaO-based high temperature CO2 sorbents – Literature review

CaO-based high temperature CO2 sorbents – Literature review

Research on the preparation of CO2 renewable sorbent from calcium-based waste: Towards enhanced biomass gasification for H2 production

The potential of industrial waste—carbide slag (CS) as a high-temperature CaO-based CO2 sorbent and its anti-sintering modification strategies are explored by examining the changes in maximum carbonation conversion (Xn,max) during multiple carbonation/calcination cycles. Although the pristine CS performs satisfactorily during initial carbonation, its Xn,max steadily declines with increasing cycle numbers due to sintering. To develop CO2 sorbent with robust, stable structures, various refractory dopants, including Al2O3, MgO, SiO2, Y2O3, and two industrial by-products rich in Al2O3 and SiO2—coal gangue (CG) and fly ash (FA)—are utilised to modify the CS. The SiO2-doped CS sorbent exhibits superior CO2 cycling adsorption capacity under identical testing conditions, attributed to the formation of Ca2SiO4 with a high Tammann temperature (TT) during high-temperature calcination. Notably, CSCG5 (CS/CG mass ratio of 95:5) presents a CO2 capture capacity comparable to CSSi2 (CS/SiO2 mass ratio of 98:2), providing a potential solution for low-cost and anti-sintering modification of CS sorbents. Relative to pure CaO or CS, the CSSi2 and CSCG5 samples exhibit superior stability in terms of H2 concentration and yield throughout the biomass cyclic hydrogen production tests. Adsorption kinetic analysis results suggest that the carbonation of CS sorbent during rapid reaction predominantly adheres to the Pseudo-first-order, Elovich, and Intra-particle diffusion models. The carbonation conversion (Xt) is excellently fitted (R2 > 0.99) by a Logistic equation considering CO2 concentration (xCO2) and carbonation temperature (Tcar). Moreover, comprehensive data analysis yields an innovative semi-empirical equation suitable for predicting the Xn,max during long-cycle carbonation/calcination processes. We also discussed the sintering and modification mechanisms of CS sorbent in depth.

Strengthening performance of Al-stabilized, CaO-based CO2 sorbent pellets by the combination of impregnated layer solution combustion and graphite-moulding

Calcium looping (CaL) process is recognized as one of the most promising CO2 capture technologies. However, the CaO-based sorbents commonly encounter serious sintering and elutriation problem during CO2 capture cycles. In this work, Al-stabilized, CaO-based sorbents were synthesized by the impregnated layer solution combustion method, and cigarette butt was selected as the porous reactive skeleton. The Al-stabilized, CaO-based sorbent containing 15 wt% of Al2O3 exhibits the highly stable CO2 capture performance, whose CO2 capture capacity after 20 cycles is almost the same as its initial capacity, ∼0.354 g/g. It is mainly attributed to the synergistic effect of thermal pretreatment (cigarette butt-assisted combustion) and incorporation of Al-based stabilizer (i.e., Ca12Al14O33) provide a stable inert framework for the synthetic sorbents, ensuring their highly stable capability for CO2 sorption during cycles. Further, graphite-moulding is a facile and energy-saving approach to make Al-stabilized, CaO-based sorbent powders into pellets to enhance its resistance to elutriation. The produced sorbent pellets (85 wt% CaO/15 wt% Al2O3) exhibit excellent long-term CO2 capture capacity (∼0.248 g/g after 49 cycles) and high compression strength of 18.47 ± 3.47 MPa. Therefore, the combined method involved of impregnated layer solution combustion and graphite-moulding is promising to produce highly efficient CaO-based CO2 sorbent pellets used in CaL.

Fabrication of robust CaO-based sorbent via entire utilization of MSW incineration bottom ash for CO2 capture

CO2 capture using CaO-based sorbent is a promising and effective CO2 mitigation strategy due to its high theoretical capacity of CO2 adsorption and potential use in large scale. The preparation of waste-derived CaO-based CO2 sorbents has received extensive interests due to its economic feasibility via waste recycling. In this work, the development of a novel robust CaO-based sorbent via the entire utilization of municipal solid waste incineration (MSWI) bottom ash (BA) was reported. Specifically, residues from the extraction of CaO from BA were added back to the extracted CaO after thermal passivation. The results showed that the addition of BA residue-derived stabilizer (BAS) promoted the homogeneous dispersion of CaO and increased the specific surface area (17.66 m2/g) of the BAS/CaO sorbents and pores in the range of 20–100 nm. Meanwhile, the small amounts of Al2O3 and SiO2 in the Ca-Si-Al-O species of BAS reacted with CaO to form Ca12Al14O33 and Ca2SiO4 during cyclic carbonation/calcination, which was beneficial to alleviate the aggregation and sintering between CaO particles and maintain porous structure of sorbents. In addition, the CO2 uptake exhibited a volcano-type variation with the increase of BAS additive amount and carbonation temperature, where the optimum carbonation temperature for BAS(20 wt%)/CaO sorbents was 700 °C. Among the various sorbents, the BAS(20 wt%)/CaO sorbent (BAS/CaO = 2/8) exhibited the most superior CO2 uptake of 0.14 gCO2/gsorbent after 20 cycles. This study provides a promising method to achieve a cost-effective CO2 capture candidate with enhanced capacity by completely reusing solid wastes, which eliminates the disposal of secondary residues that was rarely considered in previous studies.

One-pot preparation of H2-mixed CH4 fuel and CaO-based CO2 sorbent by the hydrogenation of waste clamshell/eggshell at room temperature.

The preparation of clean fuel or CO2 adsorbents using industrial and domestic garbage is an alternative way of meeting global energy needs and alleviating environmental problems. Herein, H2-mixed CH4 fuel and CaO-based CO2 sorbent were first prepared in one pot by the mechanochemical reaction of pretreated clamshell or eggshell wastes (carbon and calcium source) with calcium hydride (hydrogen source) at room temperature. In the above reactions, CH4 was the sole hydrocarbon product, and its yield reached 78.23%. The H2/CH4 ratio of the produced H2-mixed CH4 fuel was tunable according to the need by changing the reaction conditions. It is inspiring that the simultaneously formed solid CaO/carbon products were efficient CaO-based sorbents, which possessed a higher CO2 adsorption capacity (49.81-58.74wt.%) at 650°C and could maintain good adsorption stability in 30 carbonation/calcination cycles (average activity loss per cycle of only 1.6%). The three achievements of the idea are that it can simultaneously eliminate clamshell or eggshell wastes, obtain valuable clean fuel, and acquire efficient CaO-based sorbents.

Structure and surface insight into a temperature-sensitive CaO-based CO2 sorbent

Calcium looping process is a prospective strategy to remove CO2 from flue gas. However, the CO2 capture capacity of the CaO-based sorbents would be spoiled severe over the repeated cycles. In this work, a kind of NaCl-modified CaO-based sorbents were prepared by a simultaneous hydration and impregnation process and explored for cyclic CO2 capture by a simultaneous thermal analyzer. The promotion mechanism was discussed by electron microscope, X-ray diffraction, N2 physisorption, X-ray photoelectron spectroscopy, and so on. Results showed that NaCl tailored the temperature sensitivity of CaO-based sorbents, and it was a good promoter to enhance the carbonation activity of CaO-based sorbents, especially under higher temperature modes. At higher temperatures, NaCl would cover the surface of CaO, and promote the CO2 transportability in the modified sorbent, and then greatly improve the carbonation ability of the sorbents over the cycles. After 50 repeated cycles, the CO2 capture capacity of “10 NaCl/CaO” still retained at 0.288 g/g and 0.237 g/g when carbonated at 650 °C and 750 °C, which were 2.05 and 2.63 times that of “CaCO3 AR” respectively. Hence, the temperature-sensitive CaO-based sorbent promoted by NaCl shows a promising prospect for CO2 capture from flue gas.

Single-step production of hydrogen-rich syngas from toluene using multifunctional Ni-dolomite catalysts

Tar compounds such as toluene can be oxidatively converted by reforming, followed by water–gas-shift and CO2 removal to produce H2-rich syngas. We report a type of low-cost multifunctional catalysts that are capable of producing hydrogen-rich syngas from toluene in a single reactor. The multifunctional catalysts, derived from Ni-loaded dolomite, also act as oxygen carriers and CO2 sorbents. When operating under a chemical looping-type scheme at 700 °C, the catalyst containing 15 wt% NiO could produce syngas containing >70% hydrogen, with a cold-gas-efficiency of 90.3%. No sign of deactivation, coking or structural change was observed over 10 consecutive cycles of reforming and regeneration. The remarkable performance is attributed to the promotional effects arising from the formation of the MgxNi1-xO solid solution. Additionally, MgO acts as a support and provides sintering resistance to both the Ni catalyst and the CaO-based CO2 sorbent, making the multifunctional structure highly regenerable over cyclic operation.

Template-Free Synthesis of Hollow CaO/Ca2SiO4 Nanoparticle as a Cyclically Stable High-Capacity CO2 Sorbent

Cyclically stable and highly effective CaO-based CO2 sorbents are crucial to calcium looping, which is a CO2-capture technique that offers an effective and feasible way to achieve the goal of carbo...

Novel procedure of CO2 capture of the CaO sorbent activator on the reaction of one-part alkali-activated slag.

CaO derived naturally from limestone or dolomite is an inexpensive and widely available sorbent. Understanding the mechanisms of CaO carbonation at ambient temperature under the assistance of H2O is important for predicting the reaction of CaO in complex environments and designing novel CaO materials. In this study, we found that the reaction rate of alkali-activated slag is controlled by the CaCO3 layer on a partially carbonized CaO alkali activator. The size of the sorbent increased after the adsorption reaction and the physically adsorbed water in the pores accelerated the carbonation. The carbonation process was governed by CO2 diffusion. When the carbonation conversion rate reached 2–6%, the setting time increased rapidly with the increase in the carbonation rate. This is because the surface of the activator was modified by the thickened CaCO3 product layer, which increased the diffusional resistance and thus prolonged the setting time of the alkali-activated slag.

Structure and Surface Insight into a Temperature-Sensitive Cao-Based Co2 Sorbent

Structure and Surface Insight into a Temperature-Sensitive Cao-Based Co2 Sorbent

Effect of different organic compounds on the preparation of CaO-based CO2 sorbents derived from wet mixing combustion synthesis

CaO based sorbents have great potential for commercial use to capture CO 2 of power plants. In the demand of producing sorbents with better cyclic performance, CaO-based sorbents derived from different kinds of calcium precursors, containing calcium carbonate (CC-CaO), calcium gluconate monohydrate (CG-CaO), calcium citrate (CCi-CaO) and calcium acetate monohydrate (CA-CaO), were tested cyclically and compared using simultaneous thermal analyzer (STA). And further study was conducted on the sorbents modified with citric acid monohydrate and 50% gluconic acid solution by wet mixing combustion synthesis. The modified sorbents showed better performance and higher pore parameters as well as porous microstructure with more organic acid added. After 20 cycles of carbonation and calcination, the C2CCi8 (CaO: citric acid = 2:8 by mass ratio) and C2G8 (CaO: gluconic acid = 2:8 by mass ratio) sorbent possess CO 2 capture capacity of 0.45 g·g −1 (g CO 2 per g sorbents) and 0.52 g·g −1 respectively. The citric acid was more effective for modification than gluconic acid for extended 50 cycles. Furthermore, good linear relationship between CaO conversion and specific surface area as well as pore volume were determined, of which the specific surface area showed closer correlation with CaO conversion.

Facile reactivation of used CaO-based CO2 sorbent via physical treatment: Critical relationship between particle size and CO2 sorption performance

Herein we report a new and facile mechanical approach for maximizing the CO2 sorption performance of CaO sorbents, and also for recovering the sorption performance of the used (or deactivated) sorbent. We compared the physicochemical properties of CaO-based sorbents, which could change depending on their preparation methods, and determined that the CaO particle size was the most critical factor that affected their CO2 sorption performance. The CO2 sorption performance of CaO-based sorbents could be maximized via the simple physical reduction of their particle size, and this was experimentally demonstrated using ball-milling. The CO2 sorption uptake of CaO prepared using the conventional solid-state method was significantly increased to the highest ever reported level after ball-milling, and it reached 98.0% of the theoretical maximum CO2 sorption capacity. The most important application of the particle-size-dependency of CaO-based sorbents on their CO2 sorption uptake is the reactivation of used sorbents via reducing the size of the aggregated or sintered bulky particles that form after cyclic usage. The CO2 sorption uptake of the used CaO (~32.9 wt% after 10 alternative sorption–regeneration cycles) was successfully recovered (almost doubled) to ~64.7 wt% after ball-milling. This is the first time a facile mechanical-grinding-based reactivation method, which appeared to be highly efficient and directly applicable to the continuous calcium looping technology, has been developed and used.

Development of robust CaO-based sorbents from blast furnace slag for calcium looping CO2 capture

CaO-based calcium looping materials have received great interest for their use in thermochemical CO2 capture and energy storage, as well as CO2-related energy conversion processes. In this paper, the development of robust CaO-based CO2 sorbents using waste blast furnace slag (BFS) as a sustainable and environmentally benign feedstock is reported. Calcium and magnesium in the BFS can be facilely acquired through an acid extraction process, and appear in the form of CaO and MgO, which serve as a CO2 carrier and structural stabiliser, respectively, in the prepared CO2 sorbents. The BFS-derived sorbent prepared in this study exhibited a superior CO2 capture capacity of 0.37 gCO2/gsorbent with an average cyclic deactivation rate as low as 2.3%, representing a balanced performance between CO2 uptake and cyclic stability as compared to most other waste-derived, CaO-based CO2 sorbents. This study demonstrated that CaO-stabiliser interaction largely determines the CO2 capture performance of CaO-based sorbents, and the Ca/Si ratio in the BFS-derived sorbent should be effectively controlled to allow the material to capture CO2 at a high performance.

Acquiring an effective CaO-based CO2 sorbent and achieving selective methanation of CO2.

CO2 capture, utilization, and storage are promising strategies to solving the problems of superfluous CO2 or energy shortage. Here, mechanochemical reduction of CO2 by a MgH2/CaH2 mixture was first performed, by which we achieve selective methanation of CO2 and acquire an effective CaO-based CO2 sorbent, simultaneously. The selectivity of methanation is near 100% and the yield of CH4 reaches 30%. Four MgO and carbon-doped CaO-based CO2 sorbents (MgO/CaO/C, MgO/2CaO/C, MgO/4CaO/C, and MgO/8CaO/C) were formed as solid products in these reactions. Among them, the MgO/4CaO/C sorbent shows high initial adsorption amount of 59.3 wt% and low average activity loss of 1.6% after 30 cycles. This work provides a novel, well-scalable, and sustainable approach to prepare an efficient inert additive-including CaO-based CO2 sorbent and selectively convert CO2 to CH4 at the same time.

Synergy of Pore Size and Specific Surface Area on the CO₂ Sorption Performance of Nano CaO-Based Sorbents.

This study focuses on the synergy effect of pore size and specific surface area (SSA) on the carbon dioxide sorption performance. Nano CaO-based CO₂ sorbents with various pore size (15-55 nm) under similar SSA, and different SSA (14.50-48.90 m²/g) under similar pore size are prepared using selected organic templates. Results indicate that increasing the proportion of macropore in 47-96 nm could significantly improve sorbent's sorption rate and corresponding sorption capacity. Besides, sorption capacity could be also by SSA. Moreover, partial correlation analysis reveals that sorption capacity is slightly more dependent on average pore size than SSA.

Integration of a fluidised bed Ca–Cu chemical looping process in a steel mill

An integrated full system to decarbonise a steelworks plant is discussed, using high temperature Ca–Cu chemical looping reactions. A H2-enriched gas is produced through sorption enhanced water-gas-shift (SEWGS) of blast furnace gas (BFG) using a CaO-based CO2 sorbent. The resulting CaCO3 is regenerated with heat from CuO reduction with N2-free steel mill off-gases. The high temperature operation allows for an effective integration of a power steam cycle that replaces the steel mill power plant. The proposed fluidised-bed process facilitates a solids segregation step to separate the O2 solid carrier from the CO2 sorbent. The CaO-rich stream separated could be used in the steelmaking process thereby removing the lime plant. Balances of a steel mill integrated with the Ca–Cu process are solved and compared with those obtained for a reference steelworks plant with post-combustion CO2 capture through amine absorption. Using exclusively steel mill off-gases in the Ca–Cu process can reduce CO2 emissions by 30%. Moreover, the H2-gas could produce about 10% of additional iron through a Direct Reduced Iron process. In contrast, by adding natural gas for CuO reduction, almost all the BFG can be decarbonised and an overall CO2 capture efficiency in the steel plant of 92% can be achieved.

Introduction of chemically bonded zirconium oxide in CaO-based high-temperature CO2 sorbents for enhanced cyclic sorption

In this study, ZrO2 is introduced in a new form to reduce sintering and enhance the cyclic stability of CaO-based sorbents. Such a material has potential for high-temperature CO2 capture applications. Two Zr-modified CaO materials having a Ca/Zr molar ratio of 30 are prepared using the solid-state and citrate sol-gel methods. The solid-state method yields a physical mixture of CaO and ZrO2, while the citrate sol-gel method induces chemical bonding between ZrO2 and the CaO surface to form CaZrO3. The CO2 sorption uptake was significantly increased in both unmodified CaO and ZrO2-containing CaO [77.3 wt% (17.6 mol kg−1) and 73.2 wt% (16.6 mol kg−1), respectively, at 650 °C and 1 bar] when the citrate sol-gel method was used. The CaO having the chemically bonded ZrO2 reveals significantly enhanced cyclic stabilities, with an extremely high CO2 sorption uptake of 70.5 wt% (16.0 mol kg−1) on average during 10 cycles. On the other hand, the CaO containing the physically mixed ZrO2 reveals an average cyclic CO2 sorption uptake of only 37.2 wt% (8.5 mol kg−1). The chemically bonded ZrO2 is expected to be well scattered on the CaO surface and effectively cover the sorbent, resulting in the reduction of thermal sintering. In addition to cyclic stability, CO2 sorption kinetics of CaO-based sorbents can be enhanced through the citrate sol-gel method, which is resulted from the significantly reduced size of CaO particles.

Analysis of textural properties of CaO-based CO2 sorbents by ex situ USAXS

In this work, the micro-textural properties of CaO-based CO2 sorbents prepared in a thermo-gravimetric reactor were investigated by ex situ synchrotron-based Small Angle X-ray Scattering (SAXS), Ultra Small Angle X-ray Scattering (USAXS) and Wide Angle X-ray Scattering (WAXS). Six sorbent samples were produced by calcination in vacuum at different temperatures and durations of the heating step; additionally, four partially carbonated samples were produced, after calcination in vacuum, at different CaO conversions. The scattering intensity profiles were analyzed by means of the Unified Fit model. Bimodal pore size distributions (micro-pores and meso-pores) were identified and the micro-textural properties (porosity, specific surface and pore radius of gyration) of each of the two pore populations were quantified both for the calcined and for the carbonated samples. The micro-textural properties of a few calcined samples were compared with N2-adsorption measurements. For each pore population, the trends of porosity, specific surface and pore radius of gyration, in addition to the pore number density and sphericity factor, were obtained versus time during the calcination and versus the CaO conversion during the carbonation. The relevance of micro-pores with respect to macro-pores was assessed both as far as the porosity impact on the CO2 capture capacity and as far as the specific surface impact on the carbonation kinetics. For the calcined samples, the micro-textural properties of a partially calcined sample were compared with those of the completely calcined samples, and pore correlation distances were determined. For the carbonated samples, the specific surface per unit of particle volume results linearly dependent on the CaO conversion and, if expressed per unit of mass of unreacted CaO, does not change significantly for most of the CaO carbonation.